This invention relates to an electrochemical process and more particularly to an electrochemical cell in which metal-oxides can be reduced to their corresponding metals.
Electrochemical processes have been used to recover high purity metal or metals from an impure feed. Electrochemical processes have also been used to extract metals from their ores, e.g., metal-oxides. These processes typically rely on the dissolution of the metal or ore into the electrolyte and a subsequent electrolytic decomposition or selective electrotransport step. Thus they require an electrolyte in which the metal-oxide of interest is soluble. In addition, the decomposition voltage of the electrolyte should be larger than that of the metal-oxide.
In those cases where the metal-oxide has a very low solubility in the electrolyte, the reduction of the metal-oxide is typically a two-step process requiring two separate process vessels. For example in the extraction of uranium from spent nuclear oxide fuels, the first step is a chemical reduction step at 650xc2x0 C. using lithium dissolved in molten LiCl that produces uranium and Li2O. The Li2O dissolves in the molten LiCl. The second step is an electrowinning step, also at 650xc2x0 C., wherein the dissolved Li2O in the molten LiCl is electrolytically decomposed to regenerate lithium. The resulting lithium and LiCl salt with a low Li2O concentration are then recycled to the reduction vessel for reduction of the next batch of oxide fuel. A number of engineering complexities are encountered in the design of the two-step process including the transfer of molten salt and lithium at high temperatures. It would also be advantageous to replace the two process vessels with one vessel to make the process more compact and economical.
In addition, the chemical reduction of oxides in molten salts is sometimes thermodynamically constrained. In these cases either the oxides cannot be reduced at all or they can be reduced only under certain limiting conditions. The first situation can be resolved by choosing the appropriate electrolyte-reductant system. For example, some of the rare-earth oxides cannot be reduced easily with the Lixe2x80x94LiCl system but can be reduced with a Caxe2x80x94CaCl2 system. The second situation is often encountered, for example in the reduction of PuO2 in molten LiCl, the reduction can be carried out only if the Li2O concentration of the electrolyte, LiCl, is below 3.6 wt %. This limits the oxide loading of the electrolyte and as a result the oxide concentration of the electrolyte has to be carefully monitored and maintained at a low value to ensure complete reduction. This limits the amount of fuel that can be reduced in a given batch of fuel. Another consequence of limiting the oxide loading of the electrolyte is the need for more frequent transfers of molten electrolyte and liquid metal between the two process vessels.
Accordingly, it is an object of the invention to provide a process and an electrochemical cell for reducing metal-oxides to metals in a single-step using one process vessel with the ability to control the oxide concentration of the electrolyte at the desired level and to monitor the process. A significant feature of this invention is the applicability of the process for extracting a wide variety of metals by choosing an appropriate electrolyte.
Briefly, the invention is directed to an electrochemical cell for extracting metals wherein the cell includes a crucible to hold a molten electrolyte containing mobile oxide ions, a cathode consisting of a metal or ceramic basket or metal pan containing the metal-oxide or metal-oxides of interest, an anode (a non-consumable oxygen electrode), and one or more reference electrodes monitoring the electrode potentials. The anode and cathode are connected to an external power supply. Some of the advantages of the inventive process and cell are (1) it is a one-step, one-vessel process which eliminates engineering complexities associated with handling and transfer of molten salts and metals and reduces the number of components associated with a two-step process, (2) a very low level of dissolved oxide-ion concentration can be maintained in the electrolyte making it easy to reduce oxides like PuO2 and AmO2, (3) there is no formation of free-floating or excess reductant metal in the cell, (4) excellent process control through the use of reference electrodes, (5) semi-continuous process; only periodic exchange of basket or pan required, (6) environmentally friendly with oxygen being the only byproduct, (7) high-purity metals can be produced even with a starting mixture of impure metal-oxides, (8) potential to greatly reduce capital expenditure and manufacturing costs, e.g., in the processing of spent nuclear fuels since the hot-cell space requirements will be significantly reduced, and (9) it is a very versatile process and can be adapted for the extraction of a variety of metals by choosing an appropriate electrolyte.
The invention consists of certain novel features and a combination of parts hereinafter fully described, illustrated in the accompanying drawings, and particularly pointed out in the appended claims, it being understood that various changes in the details may be made without departing from the spirit, or sacrificing any of the advantages of the present invention.